The present invention relates in general to electrophotographic devices, and more particularly, to electrophotographic devices that support two or more image transfer rates and methods of operating electrophotographic devices at two or more image transfer rates.
In electrophotography, a latent image is created on the surface of an electrostatically charged photoconductive surface, e.g., a drum or belt, by exposing select portions of the photoconductive surface to laser light. Essentially, the density of the electrostatic charge on the photoconductive surface is altered in areas exposed to a laser beam relative to those areas unexposed to the laser beam. The latent electrostatic image thus created is developed into a visible image by exposing the photoconductive surface to toner, which typically contains pigment components and thermoplastic components. When so exposed, the toner is attracted to the photoconductive surface in a manner that corresponds to the electrostatic density altered by the laser beam. The toner is subsequently transferred from the photoconductive surface to a print medium such as paper, either directly or by using an intermediate transfer device. A fuser then applies heat and pressure to the print medium. The heat causes constituents including the thermoplastic components of the toner to flow into the interstices between the fibers of the medium and the fuser pressure promotes settling of the toner constituents in these voids. As the toner is cooled, it solidifies and adheres the image to the medium.
In a typical laser scanning system, a faceted rotating polygon mirror is used to sweep a laser beam across a photoconductive surface in a scan direction while the photoconductive surface advances in a process direction that is orthogonal to the scan direction. The polygon mirror speed is synchronized with the advancement of the photoconductive surface so as to achieve a desired image resolution, typically expressed in dots per inch (dpi) at a given image transfer rate, typically expressed in pages per minute (ppm). Thus, for example, to achieve a resolution of 600 dots per inch (236 dots per centimeter) in the process direction at an image transfer rate of 20 pages per minute, the photoconductive surface is operated at a speed sufficient to transfer toner images to twenty pages in one minute of time. Moreover, the polygon mirror velocity is configured to perform 600 scans across the photoconductive surface in the time it takes for the photoconductive surface to advance one inch (2.54 centimeters).
Slowing the operation of the photoconductive surface relative to a normal (full speed) operating image transfer rate can be desirable under certain circumstances. For example, slowing the photoconductive surface to one half of the full speed image transfer rate can provide double scan line addressability which, ideally, can improve the quality of the image printed on the medium. Additionally, by operating the photoconductive surface at half speed, greater time is available for fusing operations because the print medium is moving through the device at a slower speed. Relatively longer fusing times are desirable for example, when the print medium is relatively thick or where transparencies are used.
To operate satisfactorily at half speed, i.e., one half of the full speed image transfer rate, and to maintain double line addressability, the laser power needs to be reduced by one half of the full speed laser power so as to maintain output image consistency between full speed and half speed modes of printing. Unfortunately, the acceptable operating range of a typical laser diode may not allow such drastic changes in laser output power. As such, the prior art has attempted to reduce laser power output by using pulse width modulation of a full power laser beam such that the power output by the laser is reduced by one half. However, pulse width modulating a laser beam increases the complexity of the laser diode driver circuitry. Moreover, changing the duty cycle of a laser beam affects the “turn on” and “turn off” characteristics of the laser, which may affect overall consistency and print quality.